1. Field of Invention
The present invention relates to thermoplastic polyurethane elastomers. More particularly, the invention relates to a flame-resistant thermoplastic polyurethane elastomer containing a flame retardant additive package comprising a chlorinated polyethylene, a brominated aromatic compound and a compound selected from the group consisting of antimony trioxide, zinc borate, and mixtures thereof. Additionally, the invention relates to a process for the preparation of the flame retardant polyurethane elastomers and their use in the preparation of molded articles, films and cable jackets.
2. Description of Prior Art
Thermoplastic polyurethane elastomers have been known for a long time. Their commercial utility is based on their ability to combine desirable mechanical properties with the advantages of economic thermoplastic processing. A wide range of mechanical properties can be achieved by using different starting materials. An overview of thermoplastic polyurethane elastomers, their properties and applications, is given in Kunststoffe 68 (1978), pp. 819-825, or in Kautschuk, Gummi, Kunststoffe 35 (1982), pp. 568-584.
Thermoplastic polyurethane elastomers can be produced either continuously or by batch processes. The most well known of the continuous processes, the so-called sheet process and the extrusion process, are widely used on a commercial scale.
In British Pat. No. 1,057,018, for example, a prepolymer is prepared from an essentially linear hydroxyl compound and excess organic diisocyanate, fed into a mixing head through a metering pump, and mixed therein with a specified amount of a low-molecular-weight diol. The resulting reaction mixture is fed onto a conveyor belt and passed through an oven heated to from 70.degree. to 130.degree. C. until it solidifies. The reaction product is then granulated and tempered at temperatures up to 120.degree. C. for from 6 to 40 hours. The resulting product can then be processed into molded parts by the usual methods, for example, by injection molding.
In the extrusion process, which is described in German Pat. No. 20 59 570 (U.S. Pat. No. 3,642,964), the starting components are fed directly into the extruder and the reaction is performed in the extruder under the specified process conditions. The resulting polyurethane elastomer is converted into a thermoplastic directly, extruded as a strand, cooled in an inert gas atmosphere until it solidifies, and is then granulated. The disadvantage of this process is that the resulting thermoplastic polyurethane elastomer is not suitable for producing sheets, fine sections, and tubing or hose. Thermoplastic polyurethane elastomers of identical composition are transparent when made by the extrusion process, but have an opaque appearance when made by the continuous sheet process. Opaque thermoplastic polyurethane elastomers can be processed into sheets which are not subject blocking. However, transparent thermoplastic polyurethane elastomers are not suitable for this purpose.
Halogen-containing organic compounds may be incorporated in thermoplastic polyurethanes in order to improve flame retardancy. In addition, a series of metal oxides such as ZnO, B.sub.2 O.sub.3, Fe.sub.2 O.sub.3, CaO and Sb.sub.2 O.sub.3 can be added to increase flame retardancy. These compounds themselves do not exhibit a flame retarding effect, however, they do act synergistically with the organically bound halogen. The most effective system has been found to be antimony(III)oxide/halogen (see W. C. Kuryla and A. J. Lapa, Flame Retardancy of Polymeric Materials, Vol. 3, (New York: Verlag Marcel Dekker, 1975).
According to Federal Republic of Germany Pat. No. 25 24 817 (U.S. Pat. No. 3,970,716), nonflammable thermoplastic polyurethane compositions contain chlorinated polyethylene, a chlorinated cyclopentadien adduct, and optionally antimony trioxide. Federal Republic of Germany Offenlegungsschrift No. 29 01 774 describes a free flowing polyurethane elastomer concentrate containing a dye and/or an auxiliary including a flame retardant such as bromodiphenyl ether, brominated polystyrene, brominated polycarbonate and antimony trioxide.
U.S. Pat. No. 4,395,511 relates to thermoplastic polyurethanes containing a mixture of decabromodiphenyloxide and antimony trioxide as the flame retardant, optionally in combination with ammonium polyphosphate.
European Pat. No. 4175 describes a mixture of polyvinyl chloride, chlorinated polyethylene, or chlorinated paraffins and zinc oxide as well as optionally antimony trioxide to reduce smoke gas density.
The products described above all have the disadvantage that wear resistance and other mechanical properties of the thermoplastic polyurethane are adversely affected by adding the multiple-substance additives. Moreover, a particular disadvantage is that some of the low-molecular weight substances used to improve flame retardancy, for example, the chlorinated cyclopentadien adducts described in Federal Republic of Germany Pat. No. 2 524 817 and the highly brominated aromatic products such as decabromodiphenylether eventually effloresce, from the films, molded articles, tubes, hoses, and cable jackets, forming a coating on the surface of the article which is undesirable in many applications.
U.S. Pat. No. 4,397,974 generally discloses low halogen content thermoplastic polyurethanes containing a special additive mixture consisting of from 1 to 10 percent by weight antimony trioxide; 0.3 to 6 percent by weight chlorinated and/or brominated aromatic compounds having a halogen content of from 50 to 88 percent by weight; 1 to 8 percent by weight aluminum hydroxide; and 0.2 to 4 percent by weight phosphoric acid triesters; and optionally further containing up to 5 percent by weight quarternary tetraalkyl ammonium salts.
The beneficial advantages of the present invention are obtained without the necessity of including aluminum hydroxide or phosphoric acid triesters or quarternary tetraalkyl ammonium salts.
U.S. Pat. No. 4,150,066 generally discloses a flame retarding composition consisting of a mixture of (A) a first graft copolymer; (B) a second graft copolymer; (C) a chlorinated polyetheylene; (D) tetrabromobisphenol A or a derivative thereof; and (E) antimony trioxide.
U.S. Pat. No. 4,147,678 generally discloses flame retardant cross-linked polyurethane elastomer compositions. These compositions are prepared from an elastomeric cross-linkable polyurethane prepolymer; a halogen containing compound; an antimony compound; and a hexaalkoxy methyl melamine.
The beneficial advantages of the present invention are obtained without the necessity of including a hexaalkoxy methylmelamine. Further, such markedly cross-linked material is unsuitable for thermoplastic processing.
U.S. Pat. No. 3,560,441 generally discloses compositions for imparting flame retarding properties to polyolefins which comprise an admixture of antimony trioxide with an inorganic salt.
U.S. Pat. No. 3,988,296 generally discloses flame-retardant polyolefins which are said to include certain defined polyalkylenes. The brominated aromatics disclosed as fire retardants suitable for use with polyolefins effloresce or migrate from polyurethane, particularly when used with thermoplastic polyurethane.
It is known by those skilled in the art that polymers, particularly polyolefins, and additives, such as plasticizers and flame retardants, vary widely with respect to their polarity. Combinations of such materials must, therefore, be precisely matched, particularly with respect to flame retardants, to each other to prevent efflorescence or exudation of the flame retardant. The suitability of a specific flame retardant for use with polyolefins is not indicative of its suitability for use with polyurethanes in general or thermoplastic polyurethane in particular. Many examples are known by those skilled in the art of flame retardants suitable for use with polyolefins which are completely unsuitable for use with polyurethanes.